(1) Field of the Invention
The present invention relates to a method of sequential continuous-casting of different grades of steel, and particularly to a method of sequential continuous-casting of different grades of steel wherein cutting loss at the joint portion is very low and the workability of the process is excellent.
(2) Description of the Prior Art
In the case of casting different grades of molten steel by continuous process, heretofore, in general, there has been adopted such a procedure that, upon completion of continuous casting of one grade molten steel, continuous casting of another type of molten steel cannot be started until the preparatory working for continuous casting has been performed all over again. However, the period of time required for the preparatory working for continuous casting usually amounts to 60 to 90 minutes, it has not been avoidable that the casting efficiency is reduced to a considerable extent in the case of conventional working process for the different grades of molten steel.
In the case of changing over from the continuous casting of a first grade of molten steel to a second grade of molten steel that are different in constituent from each other, if the second grade of molten steel which is different in constituent from the first type of is successively poured into the mold by the conventional method without resorting to any appropriate measure, it has been known the fact that mixing is effected between the different grades of molten steel, suction caused by solidification shrinkage and bulging, and diffusion of molten steel and convection caused by difference in temperature, thus resulting in mixing between different grades of molten steel reaching a depth as deep as five to eight meters from the liquid level in a mold. As the result of said mixing action, such a slab in which different grades of molten steel are mixed together is formed in the boundary area between two grades of cast slab which cannot be used as either grade of steel and therefore should be scrapped, thus significantly decreasing the yield of molten steel. Heretofore, there have been proposed various processes for minimizing production of such mixed cast slab.
According to the invention described in the publication of Japanese Patent Application Laid-Open No. 57921/75 representing one of the typical processes, it is intended that the connecting function to connect the different grades of cast slabs to each other is performed by a connecting material such as H-steel, and the mixing occurring between the different grades of cast slabs is completely shut off by charging a coolant material such as nail scrap. However, according to this method, although these are slight differences depending on the types of continuous casting machines used, the tensile load imposed on the connecting material can reach 30 to 100 tons. And, to draw the first grade of cast slab which is continuously cast later, the drawing force is imparted to said cast slab through the connecting material connected to the lower portion of said cast slab and the first grade of cast slab connected to said connecting material. Consequently, it becomes necessary to make the end portions of the respective grades of cast slabs be solidified completely. To attain such solidification, these are presented such disadvantages that the period of time required for treatment should be extended by three to ten minutes, and moreover, such a possibility is very high that, due to notch effect breakdown occurs in a solidified shell caused to the joint portion of the connecting material interposed between the different grades of cast slabs to which said drawing force is imparted, and in turn the joint portion is broken, to thereby allow the molten steel cast to flow downwardly from the water-cooled mold.
Other examples of the prior art are disclosed in the publications of Japanese Patent Application Laid-Open Nos. 112431/76 and 30723/77, for example. These conventional techniques contemplate that one or more stages of partition plates, the center portion of which is open are arranged on the upper surface of the first grade of molten steel, and the second grade of molten steel is poured onto said partition plate or plates to thereby partition the second grade of molten steel off the first grade of molten steel. By this, the purpose has been attained to a certain extent and the cast slab in which the different grades of steel are mixed could be reduced to as short as about one meter. However, in these conventional techniques, as the partition plate or plates have the open center portions, partial mixing between the different grades of molten steel cannot be prevented from occurring, and the best result obtainable by this technique is to shorten the scope of the mixed cast slab to one meter. Further, according to these conventional techniques, there is presented such a disadvantage that, since the partition plate or plates are mounted on the upper surface of the first grade of molten steel, a solidified shell produced in the mold may be broken at a portion of said partition plate or plates, thus resulting in the so-called break-out, i.e. the molten steel leaking out of said portion when the cast slab is drawn.